The G7 Method is a printing procedure used for visually accurate color reproduction by putting emphasis on matching grayscale colorimetric measurements between processes. G7 stands for grayscale plus seven colors: the
subtractive colors typically used in printing (Cyan, Magenta, Yellow, and Black) and the
additive colors
Additive color or additive mixing is a property of a color model that predicts the appearance of colors made by coincident component lights, i.e. the perceived color can be predicted by summing the numeric representations of the component colo ...
(Red, Green, and Blue). The method is used in many applications of printing such as
offset lithography
Offset printing is a common printing technique in which the inked image is transferred (or "offset") from a plate to a rubber blanket and then to the printing surface. When used in combination with the lithographic process, which is based on ...
,
flexography, and
gravure
Rotogravure (or gravure for short) is a type of intaglio printing process, which involves engraving the image onto an image carrier. In gravure printing, the image is engraved onto a cylinder because, like offset printing and flexography
...
since it uses a one-dimensional neutral print density curve (NPDC) to match neutral tonality between two G7 calibrated printing systems. The G7 method is not a completely accurate color management system nor is it officially standardized by the
International Color Consortium
The International Color Consortium (ICC) was formed in 1993 by eight vendors in order to create an open, vendor-neutral color management system which would function transparently across all operating systems and software packages.
Overview
The ...
(ICC).
History
The G7 method was created by Don Hutcheson, chairman of the IDEAlliance GRACoL (International Digital Enterprise Alliance, General Requirements for Applications in Commercial Offset Lithography) in 2006. It was created to solve the problem of
computer to plate (CtP) printing systems and other devices printing images with different tonal value increases (TVIs), or
dot gain
Dot gain, or tonal value increase, is a phenomenon in offset lithography and some other forms of printing which causes printed material to look darker than intended. It is caused by halftone dots growing in area between the original printing film ...
s. Also, professionals in the printing industry follow the
International Organization for Standardization
The International Organization for Standardization (ISO ) is an international standard development organization composed of representatives from the national standards organizations of member countries. Membership requirements are given in Art ...
’s (ISO) principles of
half-tone
Halftone is the reprographic technique that simulates continuous-tone imagery through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect.Campbell, Alastair. The Designer's Lexicon. ©2000 Chronicle, ...
printing, called ISO 12647-2, which does not specify colorimetric values for gray balance and refers to multiple TVI curves. This results in an ambiguous definition of the print’s final ‘appearance’.
The G7 method solved this problem by creating Neutral Print Density Curves (NPDC) that related neutral density to the half-tone dot percentages of a print image rather than a TVI curve relating the input dot percentage to the output gain relative to the input percentage.
Definition
Basic Set-up
To calibrate a printing system with the G7 method, it basically entails comparing the NPDC generated by the device to the ‘ideal’ NPDC given in G7 specifications, calibrating the
Raster Image Processor
A raster image processor (RIP) is a component used in a printing system which produces a raster image also known as a bitmap. Such a bitmap is used by a later stage of the printing system to produce the printed output. The input may be a page ...
(RIP) or the driver of the device, and then repeating the NPDC comparison to check for accuracy. More specifically, one dimensional interpolation between the NPDC graph generated by the printer and the GRACoL 7 NPDC “FanGraph” must be done to generate new dot percentage aim values to be given to the device’s RIP or driver.
Colorimetry
The G7 method's emphasis on grayscale calibration is loosely based around the concept of neutralizing the camera image of a scene using a
gray card
A gray card is a middle gray reference, typically used together with a reflective light meter, as a way to produce consistent image exposure (photography), exposure and/or color in video production, film and photography.
A gray card is a flat ...
. This is done to lower the effects of lighting conditions, such as
color temperature
Color temperature is the color of light emitted by an idealized opaque, non-reflective body at a particular temperature measured in kelvins. The color temperature scale is used to categorize the color of light emitted by other light sources ...
, on the resulting image. G7 takes this further by defining percentages of cyan, magenta, and yellow ink at which the image should be gray balanced. It also defines values for a* and b* for the
Lab color space.
Most printing systems use
half-tone
Halftone is the reprographic technique that simulates continuous-tone imagery through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect.Campbell, Alastair. The Designer's Lexicon. ©2000 Chronicle, ...
s to generate images by using a series of cyan, magenta, yellow, and black dots to recreate colors. This is done by varying the size of the dots so that the human eye can spatially integrate the image's color and make it look like one uniform color.
Benefits and disadvantages
Using the G7 method for printing calibration is a manual process and is device independent, meaning it can be used for any printing devices that allows manipulation of the
raster image processor
A raster image processor (RIP) is a component used in a printing system which produces a raster image also known as a bitmap. Such a bitmap is used by a later stage of the printing system to produce the printed output. The input may be a page ...
or
computer to plate curve information that dictates how digital signals from an image is printed with ink.
However, since the G7 method relies mostly on gray balance, if the image does not contain much gray information, the calibration can be inaccurate. Also, using any offset printing may be difficult to gray balance because it involves "wet-trapping" which can lead to unpredictable values for dark tones.
Software
IDEAlink Curve
Calibration using the G7 method involves comparing two NPDCs from the G7 Fangraph to calculate new aim values that will be given to the printer's RIP files. This can be a manual process, but the IDEAlink curve software, created by IDEAlliance, does this automatically and with minimal error. Improvements to this program include Curve2 and Curve3. ''IDEAlink Curve 1.1 software is no longer commercially available''.
See the list below for systems and software that have received G7 System Certification.
IDEAlliance certifies systems and software that meet or exceed established industry tolerances for excellence and are capable of calibrating a printing device to meet the G7 grayscale definition using four 1-D curves.
G7 Certified systems/software include the following:
Agfa''
PrintTuneprocess control software/
CGS ORIS Lynx Software
EFI Fiery Color Profile Suite
FUJIFILM ColorPath Sync®
Heidelberg Prinect® Color Toolbox
CHROMiX/HutchColor Curve2
Alwan Dynamic Printer Calibration
KODAK ColorFlow
ColorGATE Productionserver / G7 Calibration Module
Konica Minolta Color Care version 2.2.1 with CurveCore Module
CHROMiX/HutchColor Curve3
Bodoni Systems pressSIGN 6 & later , pressSIGN Standard , pressSIGN Pro , pressSIGN Global Print Management
Mutoh G7 Calibrator (part of the Mutoh ColorVerify Pro/G7 process control solution for large format inkjet printers
''http://www.mutoh.com/smart-printing/color-verify''
Caldera Print Standard Verifier G7
Tucanna tCOLOR http://color.tucanna.com/
In addition to the above G7 certified products, ColorMetrix
(http://www.colormetrix.com) offers a version of its ProofPass® cloud-based process control software bundled with G7 Calibrator under license from Mutoh in the US.
References
{{Reflist
External links
Applying G7 to FlexoColorWiki - G7Printing to International Standards with G7
Printing
Color